The present resides in a hydraulic power steering for motor vehicles with a servomotor having first and second double-acting piston-cylinder units which are arranged co-axially and include a common piston rod.
Motor vehicles are generally provided with factory installed hydraulic power steerings wherein a steering handle or rather steering wheel and the steered vehicle wheels are mechanically coupled with one another. The power steering includes a servomotor with a double acting piston-cylinder unit which is operated depending on the forces and torques effective between the steering handle or steering wheel and the vehicle wheels in such a way that only limited forces are required for actuating the steering handle or turning the steering wheel. As a result, large forces or torques effective between the steering handle or steering wheel and the steered vehicle wheels are mainly taken up, or generated, by the hydraulic piston cylinder unit.
Furthermore, there are presently steering systems for normal street vehicles in the design stage which have no mechanical coupling between the steering wheel and the steered vehicle wheels. In these systems, the steering handle or steering wheel and the steered vehicle wheels are rather coupled only by way of a control system wherein only a setting means for the respective desired steering angle is operated by the steering handle or steering wheel to which angle the steered vehicle wheels are then adjusted by the control system. The control system can take additional parameters into consideration and can for example change the transmission ratio between the steering control movement of the steering handle or the steering wheel and the adjustment angle of the steered vehicle wheels depending on the vehicle speed. Furthermore, skidding movements of the vehicle can be automatically counteracted before the driver makes the appropriate steering correction or without the driver making any steering correction.
DE 29 44 833 C2 discloses a steering system which includes the features mentioned initially. It is basically a hydrostatic steering system with two parallel hydraulic operating mechanisms. It permits the elimination of the steering column which requires a relatively large amount of space and which increases the chances for a driver to be injured during an accident
EP 0 307 612 A1 discloses a power steering system wherein the steered vehicle wheels and the driver-operated steering wheel are mechanically interconnected. In addition, a hydraulic power steering unit with two parallel hydraulic circuits is provided which includes two piston-cylinder units separated by a cylinder separating wall which separates the cylinders hydraulically but through which the piston rod extends. The two cylinders are formed in a common single-piece cylindrical tube component.
DE 44 35 848 A1 discloses a servomotor consisting of a piston-cylinder unit with an annular piston member which has axial annular flanges at its inner circumference which are received in corresponding grooves formed in the piston rod.
DE 43 31 291 C1 and JP57-198168A disclose rack and pinion steering systems with a piston-cylinder units with boots disposed at the ends of the cylinders enclosing spaces which are in communication by way of an axial bore extending through the piston rod or pinion. In addition, the last mentioned reference discloses joints arranged at the opposite ends of the piston rod which cooperate with end portions of the cylinder so as to form stops for limiting travel of the piston rod.
The present invention resides in a steering system which has no need for a firm mechanical connection between a steering handle or wheel and the steered vehicle wheels. This system is similar to the control of control flaps of modern airplanes; such systems are known there under the designation "fly-by-wire" systems.
In order to provide for vehicle steering systems without mechanical coupling between the steering handle or steering wheel which are safe under any circumstances, redundancies or so called back-up arrangements must be provided which insure safe operation of the vehicle even if an essential part of the system fails. Continued travel should be possible essentially without limitations.
It is the object of the invention to provide a simple vehicle steering system without a direct mechanical coupling between the steering wheel or handle and the steered vehicle wheels wherein nevertheless continued vehicle operation is possible even if an essential part of the system fails.